Juli Clover, MacRumors:

If you swipe up on the ‌Apple Maps‌ interface where maps details are available, you can tap on a “Report” button that lets you flag an accident, a hazard, or a speed check, similar to other mapping apps like Waze.

Interestingly:

Tapping automatically flags your location with no confirmation window, so it shouldn’t be used except in a valid situation.

That does open the door to accidental flagging, but this is crowdsourcing, so it’s not likely a single report will trigger the flag.

You can also say “Hey ‌Siri‌, there’s an accident” and ‌Siri‌ will send in a report to ‌Apple Maps‌, and presumably, if enough people file reports, an accident site will show up in the maps app through the crowdsourcing.

Sounds right. Nice to see Siri interface.